Molecular characterization of two human autoantigens: unique cDNAs encoding 95- and 160-kD proteins of a putative family in the Golgi complex

The function of Golgi apparatus in LRRK2-associated Parkinson's disease

Parkinson's disease (PD) is a chronic neurodegenerative disease associated with the intracellular organelles. Leucine-rich repeat kinase 2 (LRRK2) is a large multi-structural domain protein, and mutation in LRRK2 is associated with PD. LRRK2 regulates intracellular vesicle transport and function of organelles, including Golgi and lysosome. LRRK2 phosphorylates a group of Rab GTPases, including Rab29, Rab8, and Rab10. Rab29 acts in a common pathway with LRRK2. Rab29 has been shown to recruit LRRK2 to the Golgi complex (GC) to stimulate LRRK2 activity and alter the Golgi apparatus (GA). Interaction between LRRK2 and Vacuolar protein sorting protein 52 (VPS52), a subunit of the Golgi-associated retrograde protein (GARP) complex, mediates the function of intracellular soma trans-Golgi network (TGN) transport. VPS52 also interacts with Rab29. Knockdown of VPS52 leads to the loss of LRRK2/Rab29 transported to the TGN. Rab29, LRRK2, and VPS52 work together to regulate functions of the GA, which is associated with PD. We highlight recent advances in the roles of LRRK2, Rabs, VPS52, and other molecules, such as Cyclin-dependent kinase 5 (CDK5) and protein kinase C (PKC) in the GA, and discuss their possible association with the pathological mechanisms of PD.

An Autoantigen Atlas From Human Lung HFL1 Cells Offers Clues to Neurological and Diverse Autoimmune Manifestations of COVID-19

COVID-19 is accompanied by a myriad of both transient and long-lasting autoimmune responses. Dermatan sulfate (DS), a glycosaminoglycan crucial for wound healing, has unique affinity for autoantigens (autoAgs) from apoptotic cells. DS-autoAg complexes are capable of stimulating autoreactive B cells and autoantibody production. We used DS-affinity proteomics to define the autoantigen-ome of lung fibroblasts and bioinformatics analyses to study the relationship between autoantigenic proteins and COVID-induced alterations. Using DS-affinity, we identified an autoantigen-ome of 408 proteins from human HFL1 cells, at least 231 of which are known autoAgs. Comparing with available COVID data, 352 proteins of the autoantigen-ome have thus far been found to be altered at protein or RNA levels in SARS-CoV-2 infection, 210 of which are known autoAgs. The COVID-altered proteins are significantly associated with RNA metabolism, translation, vesicles and vesicle transport, cell death, supramolecular fibrils, cytoskeleton, extracellular matrix, and interleukin signaling. They offer clues to neurological problems, fibrosis, smooth muscle dysfunction, and thrombosis. In particular, 150 altered proteins are related to the nervous system, including axon, myelin sheath, neuron projection, neuronal cell body, and olfactory bulb. An association with the melanosome is also identified. The findings from our study illustrate a connection between COVID infection and autoimmunity. The vast number of COVID-altered proteins with high intrinsic propensity to become autoAgs offers an explanation for the diverse autoimmune complications in COVID patients. The variety of autoAgs related to mRNA metabolism, translation, and vesicles suggests a need for long-term monitoring of autoimmunity in COVID. The COVID autoantigen atlas we are establishing provides a detailed molecular map for further investigation of autoimmune sequelae of the pandemic, such as "long COVID" syndrome.

Summary Sentence

An autoantigen-ome by dermatan sulfate affinity from human lung HFL1 cells may explain neurological and autoimmune manifestations of COVID-19.

Getting Sugar Coating Right! The Role of the Golgi Trafficking Machinery in Glycosylation

The Golgi is the central organelle of the secretory pathway and it houses the majority of the glycosylation machinery, which includes glycosylation enzymes and sugar transporters. Correct compartmentalization of the glycosylation machinery is achieved by retrograde vesicular trafficking as the secretory cargo moves forward by cisternal maturation. The vesicular trafficking machinery which includes vesicular coats, small GTPases, tethers and SNAREs, play a major role in coordinating the Golgi trafficking thereby achieving Golgi homeostasis. Glycosylation is a template-independent process, so its fidelity heavily relies on appropriate localization of the glycosylation machinery and Golgi homeostasis. Mutations in the glycosylation enzymes, sugar transporters, Golgi ion channels and several vesicle tethering factors cause congenital disorders of glycosylation (CDG) which encompass a group of multisystem disorders with varying severities. Here, we focus on the Golgi vesicle tethering and fusion machinery, namely, multisubunit tethering complexes and SNAREs and their role in Golgi trafficking and glycosylation. This review is a comprehensive summary of all the identified CDG causing mutations of the Golgi trafficking machinery in humans.

A Master Autoantigen-ome Links Alternative Splicing, Female Predilection, and COVID-19 to Autoimmune Diseases

Chronic and debilitating autoimmune sequelae pose a grave concern for the post-COVID-19 pandemic era. Based on our discovery that the glycosaminoglycan dermatan sulfate (DS) displays peculiar affinity to apoptotic cells and autoantigens (autoAgs) and that DS-autoAg complexes cooperatively stimulate autoreactive B1 cell responses, we compiled a database of 751 candidate autoAgs from six human cell types. At least 657 of these have been found to be affected by SARS-CoV-2 infection based on currently available multi-omic COVID data, and at least 400 are confirmed targets of autoantibodies in a wide array of autoimmune diseases and cancer. The autoantigen-ome is significantly associated with various processes in viral infections, such as translation, protein processing, and vesicle transport. Interestingly, the coding genes of autoAgs predominantly contain multiple exons with many possible alternative splicing variants, short transcripts, and short UTR lengths. These observations and the finding that numerous autoAgs involved in RNA-splicing showed altered expression in viral infections suggest that viruses exploit alternative splicing to reprogram host cell machinery to ensure viral replication and survival. While each cell type gives rise to a unique pool of autoAgs, 39 common autoAgs associated with cell stress and apoptosis were identified from all six cell types, with several being known markers of systemic autoimmune diseases. In particular, the common autoAg UBA1 that catalyzes the first step in ubiquitination is encoded by an X-chromosome escape gene. Given its essential function in apoptotic cell clearance and that X-inactivation escape tends to increase with aging, UBA1 dysfunction can therefore predispose aging women to autoimmune disorders. In summary, we propose a model of how viral infections lead to extensive molecular alterations and host cell death, autoimmune responses facilitated by autoAg-DS complexes, and ultimately autoimmune diseases. Overall, this master autoantigen-ome provides a molecular guide for investigating the myriad of autoimmune sequalae to COVID-19 and clues to the rare but reported adverse effects of the currently available COVID vaccines.

The role of the Golgi apparatus in disease (Review)

The Golgi apparatus is known to underpin many important cellular homeostatic functions, including trafficking, sorting and modifications of proteins or lipids. These functions are dysregulated in neurodegenerative diseases, cancer, infectious diseases and cardiovascular diseases, and the number of disease-related genes associated with Golgi apparatus is on the increase. Recently, many studies have suggested that the mutations in the genes encoding Golgi resident proteins can trigger the occurrence of diseases. By summarizing the pathogenesis of these genetic diseases, it was found that most of these diseases have defects in membrane trafficking. Such defects typically result in mislocalization of proteins, impaired glycosylation of proteins, and the accumulation of undegraded proteins. In the present review, we aim to understand the patterns of mutations in the genes encoding Golgi resident proteins and decipher the interplay between Golgi resident proteins and membrane trafficking pathway in cells. Furthermore, the detection of Golgi resident protein in human serum samples has the potential to be used as a diagnostic tool for diseases, and its central role in membrane trafficking pathways provides possible targets for disease therapy. Thus, we also introduced the clinical value of Golgi apparatus in the present review.

An Autoantigen Atlas from Human Lung HFL1 Cells Offers Clues to Neurological and Diverse Autoimmune Manifestations of COVID-19

COVID-19 is accompanied by a myriad of both transient and long-lasting autoimmune responses. Dermatan sulfate (DS), a glycosaminoglycan crucial for wound healing, has unique affinity for autoantigens (autoAgs) from apoptotic cells. DS-autoAg complexes are capable of stimulating autoreactive B cells and autoantibody production. Using DS affinity, we identified an autoantigenome of 408 proteins from human fetal lung fibroblast HFL11 cells, at least 231 of which are known autoAgs. Comparing with available COVID data, 352 proteins of the autoantigenome have thus far been found to be altered at protein or RNA levels in SARS-Cov-2 infection, 210 of which are known autoAgs. The COVID-altered proteins are significantly associated with RNA metabolism, translation, vesicles and vesicle transport, cell death, supramolecular fibrils, cytoskeleton, extracellular matrix, and interleukin signaling. They offer clues to neurological problems, fibrosis, smooth muscle dysfunction, and thrombosis. In particular, 150 altered proteins are related to the nervous system, including axon, myelin sheath, neuron projection, neuronal cell body, and olfactory bulb. An association with the melanosome is also identified. The findings from our study illustrate a strong connection between viral infection and autoimmunity. The vast number of COVID-altered proteins with propensity to become autoAgs offers an explanation for the diverse autoimmune complications in COVID patients. The variety of autoAgs related to mRNA metabolism, translation, and vesicles raises concerns about potential adverse effects of mRNA vaccines. The COVID autoantigen atlas we are establishing provides a detailed molecular map for further investigation of autoimmune sequelae of the pandemic.

Anti-rods/rings autoantibody and IMPDH filaments: an update after fifteen years of discovery

Autoantibodies to unknown subcellular rod and ring-shaped structures were first discovered in sera from hepatitis C patients in 2005. Early studies showed a strong association between these anti-rods/rings antibodies (anti-RR) and the standard of care interferon-α plus ribavirin combination therapy (IFN/RBV), suggesting that anti-RR are drug-induced autoantibodies. In the context of hepatitis C, anti-RR have been linked with relapse from or lack of response to IFN/RBV in some patient cohorts. However, examples of anti-RR in other diseases and healthy individuals have also been reported over the years, although anti-RR remains a rare autoantibody response in general. The advent of new direct-acting antiviral drugs for chronic hepatitis C and studies of anti-RR from different parts of the world are also beginning to change the perception of anti-RR. The nucleotide biosynthetic enzyme inosine monophosphate dehydrogenase (IMPDH) has been identified as the major autoantigen recognized by anti-RR. Coincidentally, the assembly of IMPDH into micron-scale rod and ring-shaped structures was discovered around the same time as anti-RR. Knowledge of the fundamental biological properties and cellular functions of these structures, referred to as "IMPDH filaments" by cell biologists, has advanced in parallel to anti-RR antibodies. Recent studies have revealed that IMPDH filament assembly is a mechanism to prevent feedback inhibition of IMPDH and is therefore important for the increased nucleotide production required in hyperproliferating cells, like activated T cells. Fifteen years later, we review the history and current knowledge in both the anti-RR autoantibody and IMPDH filament fields. TAKE-HOME MESSAGE: Anti-rods/rings are recognized as an example of a drug-induced autoantibody in hepatitis C patients treated with interferon and ribavirin, although new studies suggest anti-rods/rings may be detected in other contexts and may depend on unknown environmental or genetic factors in different populations. Recent data suggest that the assembly of IMPDH into rod and ring structures, the targets of anti-rods/rings autoantibody, is a mechanism for hyperproliferating cells, like activated T cells, to maintain increased guanine nucleotide levels to support rapid cell division.

Golgins

Muschalik and Munro introduce golgins and their roles as vesicle tethers and scaffolds at the Golgi.

Form and function of the Golgi apparatus: scaffolds, cytoskeleton and signalling

In addition to the classical functions of the Golgi in membrane transport and glycosylation, the Golgi apparatus of mammalian cells is now recognised to contribute to the regulation of a range of cellular processes, including mitosis, DNA repair, stress responses, autophagy, apoptosis and inflammation. These processes are often mediated, either directly or indirectly, by membrane scaffold molecules, such as golgins and GRASPs which are located on Golgi membranes. In many cases, these scaffold molecules also link the actin and microtubule cytoskeleton and influence Golgi morphology. An emerging theme is a strong relationship between the morphology of the Golgi and regulation of a variety of signalling pathways. Here, we review the molecular regulation of the morphology of the Golgi, especially the role of the golgins and other scaffolds in the interaction with the microtubule and actin networks. In addition, we discuss the impact of the modulation of the Golgi ribbon in various diseases, such as neurodegeneration and cancer, to the pathology of disease.

At the ends of their tethers! How coiled-coil proteins capture vesicles at the Golgi

Cells face a complex problem: how to transfer lipids and proteins between membrane compartments in an organized, timely fashion. Indeed, many thousands of membrane and secretory proteins must traffic out of the ER to different organelles to function, while others are retrieved from the plasma membrane having fulfilled their roles [Nat. Rev. Mol. Cell Biol. (2013) 14, 382-392]. This process is highly dynamic and failure to target cargo accurately leads to catastrophic consequences for the cell, as is clear from the numerous human diseases associated with defects in membrane trafficking [Int. J. Mol. Sci. (2013) 14, 18670-18681; Traffic (2000) 1, 836-851]. How then does the cell organize this enormous transfer of material in its crowded internal environment? And how specifically do vesicles carrying proteins and lipids recognize and fuse with the correct compartment?

'Rod and ring' formation from IMP dehydrogenase is regulated through the one-carbon metabolic pathway

'Rods and rings' (RRs) are conserved, non-membrane-bound intracellular polymeric structures composed, in part, of inosine monophosphate dehydrogenase (IMPDH), a key enzyme leading to GMP and GTP biosynthesis. RR formation is induced by IMPDH inhibitors as well as glutamine deprivation. They also form upon treatment of cells with glutamine synthetase inhibitors. We now report that depriving cells of serine and glycine promotes RR formation, and we have traced these effects to dihydrofolate reductase (DHFR) and serine hydroxymethyltransferase-2 (SHMT2), pivotal enzymes in one-carbon metabolism and nucleotide biosynthesis. RR assembly is likewise induced upon DHFR inhibition by methotrexate or aminopterin as well as siRNA-mediated knockdown of DHFR or SHMT2. Because RR assembly occurs when guanine nucleotide biosynthesis is inhibited, and because RRs rapidly disassemble after the addition of guanine nucleotide precursors, RR formation might be an adaptive homeostatic mechanism, allowing IMPDH to sense changes in the one-carbon folate pathway.

A candidate gene study reveals association between a variant of the Peroxisome Proliferator-Activated Receptor Gamma (PPAR-γ) gene and systemic sclerosis

Introduction

The multifunctional nuclear receptor peroxisome proliferator-activated receptor gamma (PPAR-γ) has potent anti-fibrotic effects, and its expression and activity are impaired in patients with systemic sclerosis (SSc). We investigated PPAR-γ gene (PPARG) single nucleotide polymorphisms (SNPs) associated with SSc.

Methods

Tag SNPs spanning PPARG were genotyped in a European ancestry US discovery cohort comprising 152 SSc patients and 450 controls, with replication of our top signal in a European cohort (1031 SSc patients and 1014 controls from France). Clinical parameters and disease severity were analyzed to evaluate clinical associations with PPARG variants.

Results

In the discovery cohort, a single PPARG intronic SNP (rs10865710) was associated with SSc (p = 0.010; odds ratio = 1.52 per C allele, 95% confidence interval 1.10-2.08). This association was replicated in the French validation cohort (p = 0.052; odds ratio = 1.16 per C allele, 95% confidence interval 1.00-1.35). Meta-analysis of both cohorts indicated stronger evidence for association (p = 0.002; odds ratio = 1.22 per C allele, 95% confidence interval 1.07-1.40). The rs10865710 C allele was also associated with pulmonary arterial hypertension in the French SSc cohort (p = 0.002; odds ratio = 2.33 per C allele, 95% confidence interval 1.34-4.03).

Conclusions

A PPARG variant is associated with susceptibility to SSc, consistent with a role of PPAR-γ in the pathogenesis of SSc.

Electronic supplementary material

The online version of this article (doi:10.1186/s13075-015-0641-2) contains supplementary material, which is available to authorized users.

Glutamine deprivation initiates reversible assembly of mammalian rods and rings

Rods and rings (RR) are protein assemblies composed of cytidine triphosphate synthetase type 1 (CTPS1) and inosine monophosphate dehydrogenase type 2 (IMPDH2), key enzymes in CTP and GTP biosynthesis. Small-molecule inhibitors of CTPS1 or IMPDH2 induce RR assembly in various cancer cell lines within 15 min to hours. Since glutamine is an essential amide nitrogen donor in these nucleotide biosynthetic pathways, glutamine deprivation was examined to determine whether it leads to RR formation. HeLa cells cultured in normal conditions did not show RR, but after culturing in media lacking glutamine, short rods (<2 μm) assembled after 24 h, and longer rods (>5 μm) formed after 48 h. Upon supplementation with glutamine or guanosine, these RR underwent almost complete disassembly within 15 min. Inhibition of glutamine synthetase with methionine sulfoximine also increased RR assembly in cells deprived of glutamine. Taken together, our data support the hypothesis that CTP/GTP biosynthetic enzymes polymerize to form RR in response to a decreased intracellular level of glutamine. We speculate that rod and ring formation is an adaptive metabolic response linked to disruption of glutamine homeostasis.

Structural biology of Arf and Rab GTPases' effector recruitment and specificity

Arf and Rab proteins, members of small GTPases superfamily, localize to specific subcellular compartments and regulate intracellular trafficking. To carry out their cellular functions, Arfs/Rabs interact with numerous and structurally diverse effector proteins. Over the years, a number of Arf/Rab:effector complexes have been crystallized and their structures reveal shared binding modes including α-helical packing, β-β complementation, and heterotetrameric assemblies. We review available structural information and provide a framework for in-depth analysis of complexes. The unifying features that we identify are organized into a classification scheme for different modes of Arf/Rab:effector interactions, which includes "all-α-helical," "mixed α-helical," "β-β zipping," and "bivalent" modes of binding. Additionally, we highlight structural determinants that are the basis of effector specificity. We conclude by expanding on functional implications that are emerging from available structural information under our proposed classification scheme.

Human autoantibodies as reagents in biomedical research

Abstract  Autoantibodies are typically associated with autoimmune diseases. In some instances the association of specific autoantibodies to a specific autoimmune disease have made their detection invaluable in clinical diagnosis. However, certain autoantibodies show no specific disease association and therefore have limited clinical utility. Nevertheless, autoantibodies are powerful tools for identification, characterization, and functional studies of their cognate antoantigens. In addition, the study of autoantibodies and their cognate autoantigens in human disease and in experimental animal models can provide valuable insight into disease mechanisms and the factors that ameliorate or reverse disease. This review will focus on three specific sets of autoantibodies, which were initially selected for investigation purely on the basis of their novel patterns of reactivity. These were observed when they were applied to a diagnostic HEp-2 test slide for antinuclear antibody detection by indirect immunofluorescence. The target autoantigens were identified as the trans-Golgi network protein GOLGA4 (Golgin 245 or p230), the early endosome antigen-1 (EEA1) and a yet to be identified and fully characterized phosphoepitope(s) restricted to chromosomal arms of condensed mitotic/meiotic chromosomes (MCA1). This laboratory has exploited sera which are reactive to these autoantigens for their identification and characterization, and in functional studies. This review highlights the uses of autoantibodies that may have limited diagnostic or prognostic utility, but are nonetheless novel reagents in the prosecution of molecular cell biology.

New point mutation in Golga3 causes multiple defects in spermatogenesis

Mice with repro27 exhibit fully penetrant male-specific infertility associated with a nonsense mutation in the golgin subfamily A member 3 gene (Golga3). GOLGA3 is a Golgi complex-associated protein implicated in protein trafficking, apoptosis, positioning of the Golgi and spermatogenesis. In repro27 mutant mice, a point mutation in exon 18 of the Golga3 gene that inserts a pre-mature termination codon leads to an absence of GOLGA3 protein expression. GOLGA3 protein was undetectable in the brain, heart and liver in both mutant and control mice. Although spermatogenesis in Golga3(repro27) mutant mice appears to initiate normally, development is disrupted in late meiosis during the first wave of spermatogenesis, leading to significant germ cell loss between 15 and 18 days post-partum (dpp). Terminal Deoxynucleotidyl Transferase dUTP-mediated Nick End Labeling analysis showed elevated DNA fragmentation in meiotic germ cells by 12 dpp, suggesting apoptosis as a mechanism of germ cell loss. The few surviving post-meiotic round spermatids exhibited abnormal spermiogenesis with defects in acrosome formation, head and tail development and extensive vacuolization in the seminiferous epithelium. Analysis of epididymal spermatozoa showed significantly low sperm concentration and motility and in vitro fertilization with mutant spermatozoa was unsuccessful. Golga3(repro27) mice lack GOLGA3 protein and thus provide an in vivo tool to aid in deciphering the role of GOLGA3 in Golgi complex positioning, cargo trafficking and apoptosis signalling in male germ cells.

Oligomerization of rab/effector complexes in the regulation of vesicle trafficking

Rabs comprise the largest member of the Ras superfamily of small GTPases with over 60 proteins in mammals and 11 proteins in yeast. Like all small GTPases, Rabs oscillate between an inactive GDP-bound conformation and an active GTP-bound state that is tethered to lipid membranes via a C-terminal prenylation site on conserved cysteine residues. In their active state, Rabs regulate various aspects of membrane trafficking, including vesicle formation, transport, docking, and fusion. The critical element of biological activity is the recruitment of cytosolic effector proteins to specific endomembranes by active Rabs. The importance of Rabs in cellular processes is apparent from their links to genetic disorders, immunodeficiency, cancer, and pathogen invasion. During the last decade, numerous structures of complexes have shed light on the molecular basis for Rab/effector specificity and their topological organization on subcellular membranes. Here, I review the known structures of Rab/effector complexes and their modes of oligomerization. This is followed by a brief discussion on the thermodynamics of effector recruitment, which has not been documented sufficiently in previous reviews. A summary of diseases associated with Rab/effector trafficking pathways concludes this chapter.

Acetylcholine receptors enable the transport of rapsyn from the Golgi complex to the plasma membrane

The accumulation of acetylcholine receptors (AChRs) at nerve terminals is critical for signal transmission at the neuromuscular junction, and rapsyn is essential for this process. Previous studies suggest that AChRs might direct rapsyn self-clusters to the synapse. In vivo experiments with fluorescently tagged AChR or rapsyn in zebrafish larvae revealed that rapsyn self-clusters separate from AChRs did not exist before synapse formation. Examination of rapsyn in the AChR-less mutant sofa potato revealed that rapsyn in the absence of AChR was localized in the Golgi complex. Expression of muscle-type AChR in sofa potato restored synaptic clustering of rapsyn, while neuronal type AChR had no effect. To determine whether this requirement of protein interaction is reciprocal, we examined the mutant twitch once, which has a missense mutation in rapsyn. While the AChRs distributed nonsynaptically on the plasma membrane in twitch once, mutant rapsyn was retained in the Golgi complex. We conclude that AChRs enable the transport of rapsyn from the Golgi complex to the plasma membrane through a molecule-specific interaction.

Induction of cytoplasmic rods and rings structures by inhibition of the CTP and GTP synthetic pathway in mammalian cells

Background

Cytoplasmic filamentous rods and rings (RR) structures were identified using human autoantibodies as probes. In the present study, the formation of these conserved structures in mammalian cells and functions linked to these structures were examined.

Methodology/Principal Findings

Distinct cytoplasmic rods (∼3–10 µm in length) and rings (∼2–5 µm in diameter) in HEp-2 cells were initially observed in immunofluorescence using human autoantibodies. Co-localization studies revealed that, although RR had filament-like features, they were not enriched in actin, tubulin, or vimentin, and not associated with centrosomes or other known cytoplasmic structures. Further independent studies revealed that two key enzymes in the nucleotide synthetic pathway cytidine triphosphate synthase 1 (CTPS1) and inosine monophosphate dehydrogenase 2 (IMPDH2) were highly enriched in RR. CTPS1 enzyme inhibitors 6-diazo-5-oxo-L-norleucine and Acivicin as well as the IMPDH2 inhibitor Ribavirin exhibited dose-dependent induction of RR in >95% of cells in all cancer cell lines tested as well as mouse primary cells. RR formation by lower concentration of Ribavirin was enhanced in IMPDH2-knockdown HeLa cells whereas it was inhibited in GFP-IMPDH2 overexpressed HeLa cells. Interestingly, RR were detected readily in untreated mouse embryonic stem cells (>95%); upon retinoic acid differentiation, RR disassembled in these cells but reformed when treated with Acivicin.

Conclusions/Significance

RR formation represented response to disturbances in the CTP or GTP synthetic pathways in cancer cell lines and mouse primary cells and RR are the convergence physical structures in these pathways. The availability of specific markers for these conserved structures and the ability to induce formation in vitro will allow further investigations in structure and function of RR in many biological systems in health and diseases.

Intra-Golgi formation of IgM-glycosaminoglycan complexes promotes Ig deposition

Immune complexes arise from interactions between secreted Ab and Ags in the surrounding milieu. However, it is not known whether intracellular Ag-Ab interactions also contribute to the formation of extracellular immune complexes. In this study, we report that certain murine B cell hybridomas accumulate intracellular IgM and release large, spherical IgM complexes. The complexes (termed "spherons") reach 2 μm in diameter, detach from the cell surface, and settle out of solution. The spherons contain IgM multimers that incorporate the J chain and resist degradation by endoglycosidase H, arguing for IgM passage through the Golgi. Treatment of cells with inhibitors of proteoglycan synthesis, or incubation of spherons with chondroitinase ABC, degrades spherons, indicating that spheron formation and growth depend on interactions between IgM and glycosaminoglycans. This inference is supported by direct binding of IgM to heparin and hyaluronic acid. We conclude that, as a consequence of IgM binding to glycosaminoglycans, multivalent IgM-glycan complexes form in transit of IgM to the cell surface. Intra-Golgi formation of immune complexes could represent a new pathogenic mechanism for immune complex deposition disorders.

The golgin coiled-coil proteins of the Golgi apparatus

A number of long coiled-coil proteins are present on the Golgi. Often referred to as "golgins," they are well conserved in evolution and at least five are likely to have been present in the last common ancestor of all eukaryotes. Individual golgins are found in different parts of the Golgi stack, and they are typically anchored to the membrane at their carboxyl termini by a transmembrane domain or by binding a small GTPase. They appear to have roles in membrane traffic and Golgi structure, but their precise function is in most cases unclear. Many have binding sites for Rab family GTPases along their length, and this has led to the suggestion that the golgins act collectively to form a tentacular matrix that surrounds the Golgi to capture Rab-coated membranes in the vicinity of the stack. Such a collective role might explain the lack of cell lethality seen following loss of some of the genes in human familial conditions or mouse models.

Identification of a common autoantigenic epitope of protein disulfide isomerase, golgin-160 and voltage-gated potassium channel in type 1 diabetes

A common epitope of proteins golgin-160, voltage-gated potassium channel and disulfide isomerase was identified by screening with autoantibodies of a type 1 diabetic (T1D) patient a lambdaUni-Zap cDNA library from human diabetic islets. The significance of the identified autoantigens to the disease pathogenesis remains to be elucidated.

Emerging new roles of GM130, a cis-Golgi matrix protein, in higher order cell functions

GM130 is a peripheral membrane protein strongly attached to the Golgi membrane and is isolated from the detergent and salt resistant Golgi matrix. GM130 is rich in coiled-coil structures and predicted to take a rod-like shape. Together with p115, giantin, and GRASP65, GM130 facilitates vesicle fusion to the Golgi membrane as a vesicle "tethering factor". GM130 is also involved in the maintenance of the Golgi structure and plays a major role in the disassembly and reassembly of the Golgi apparatus during mitosis. Emerging evidence suggests that GM130 is involved in the control of glycosylation, cell cycle progression, and higher order cell functions such as cell polarization and directed cell migration. This creates the potential for novel Golgi-targeted drugs and treatments for various diseases including glycosylation defects, immune diseases, and cancer.

Acyl-coenzyme A binding domain containing 3 (ACBD3; PAP7; GCP60): an emerging signaling molecule

Golgi body-mediated signaling has been linked to its fragmentation and regeneration during the mitotic cycle of the cell. During this process, Golgi-resident proteins are released to the cytosol and interact with other signaling molecules to regulate various cellular processes. Acyl-coenzyme A binding domain containing 3 protein (ACBD3) is a Golgi protein involved in several signaling events. ACBD3 protein was previously known as peripheral-type benzodiazepine receptor and cAMP-dependent protein kinase associated protein 7 (PAP7), Golgi complex-associated protein of 60kDa (GCP60), Golgi complex-associated protein 1 (GOCAP1), and Golgi phosphoprotein 1 (GOLPH1). In this review, we present the gene ontology of ACBD3, its relations to other Acyl-coenzyme A binding domain containing (ACBD) proteins, and its biological function in steroidogenesis, apoptosis, neurogenesis, and embryogenesis. We also discuss the role of ACBD3 in asymmetric cell division and cancer. New findings about ACBD3 may help understand this newly characterized signaling molecule and stimulate further research into its role in molecular endocrinology, neurology, and stem cell biology.

Golgins and GRASPs: holding the Golgi together

The GRASP and golgin families of proteins have emerged as key components of the Golgi apparatus, with major roles in both the structural organisation of this organelle and the trafficking that occurs there. Both types of protein participate in membrane tethering events that occur upstream of membrane fusion as well as contributing to the structural scaffold that defines Golgi architecture, referred to as the Golgi matrix. The importance of these proteins is highlighted by their targeting in mitosis, apoptosis, and pathogenic infections that cause dramatic structural and functional reorganisation of the Golgi apparatus. In this review we will discuss our current understanding of GRASP and golgin function, highlighting some of the common themes that have emerged as well as describing previously unsuspected roles for these proteins in various cellular processes.

Role of vesicle tethering factors in the ER-Golgi membrane traffic

Tethers are a diverse group of loosely related proteins and protein complexes grouped into three families based on structural and functional similarities. A well-accepted role for tethering factors is the initial attachment of transport carriers to acceptor membranes prior to fusion. However, accumulating evidence indicates that tethers are more than static bridges. Tethers have been shown to interact with components of the fusion machinery and with components involved in vesicle formation. Tethers belonging to the three families act at the same stage of traffic, suggesting that they mediate distinct events during vesicle tethering. Thus, multiple tether-facilitated events are required to provide selectivity to vesicle fusion. In this review, we highlight findings that support this model.

Redefining the Scl-70 indirect immunofluorescence pattern: autoantibodies to DNA topoisomerase I yield a specific compound immunofluorescence pattern

Objectives. To report on a novel IIF pattern specifically associated with antibodies to DNA topo I.

Methods. A novel compound IF pattern, designated Scl-70 pattern, was characterized in routine ANA-HEp-2 IIF screening. Within the last 3 years, all serum samples presenting the Scl-70 pattern at the ANA-HEp2 IIF screening were tested for anti-topo I reactivity. Conversely, 16 serum samples with known anti-topo I reactivity and affinity-purified anti-topo I antibody preparations were tested for the Scl-70 pattern.

Results. The Scl-70 pattern comprised the staining of five cellular regions: nucleus, nucleolus and cytoplasm in interphase cells; nucleolar organizing region (NOR) and chromosomes in mitotic cells. All 81 serum samples selected as Scl-70 pattern reacted with topo I. All 16 anti-topo I samples and antibody preparations reproduced the Scl-70 pattern. This compound IF pattern was consistently observed in different commercial HEp-2 cell slides and in home-made slides with HEp-2 cells and human fibroblasts fixed with alternative protocols. Double IIF experiments demonstrated the co-localization of topo I and human upstream binding factor at the NOR.

Conclusions. The Scl-70 pattern belongs to the group of compound IF patterns that hold strong association with the respective autoantibody specificities, such as that observed with centromere protein F (CENP-F) and nuclear mitotic apparatus-1 (NuMA-1) protein. The identification of the Scl-70 pattern at routine ANA-HEp-2 IIF screening may lead to implementation of specific tests for the identification of anti-topo I antibodies. In addition, the Scl-70 pattern outlines cellular domains other than those previously reported for topo I, which is of interest for further understanding the roles of this enzyme in cell biology.

Co-clustering of Golgi complex and other cytoplasmic organelles to crescentic region of half-moon nuclei during apoptosis

Early apoptosis is defined by stereotypic morphological changes, especially evident in the nucleus, where chromatin condenses and compacts, and assumes a globular, half-moon or crescent-shaped morphology. Accumulating evidence suggests that cytoplasmic organelles such as mitochondria and the Golgi complex are major sites of integration of pro-apoptotic signaling. In this study, cytoplasmic organelles including Golgi complex, mitochondria, endosomes, lysosomes, and peroxisomes were shown to condense at the same unique region adjacent to the crescentic nucleus during a relatively early stage of apoptosis induced by staurosporine or other agents. The co-clustering phenomenon may be caused by shrinkage of cytoplasm during apoptosis although cytoskeletal markers actin and tubulin were not condensed and appeared excluded. These data suggest the co-clustering of cytoplasmic organelles plays an interesting role during the progression of the apoptotic process. It is possible that modification of pro-apoptotic proteins may arise as a result of the interplay of these cytoplasmic organelles.

Characterization of the topology and functional domains of RKTG

RKTG (Raf kinase trapping to Golgi) is exclusively localized at the Golgi apparatus and functions as a spatial regulator of Raf-1 kinase by sequestrating Raf-1 to the Golgi. Based on the structural similarity with adiponectin receptors, RKTG was predicted to be a seven-transmembrane protein with a cytosolic N-terminus, distinct from classical GPCRs (G-protein-coupled receptors). We analysed in detail the topology and functional domains of RKTG in this study. We determined that the N-terminus of RKTG is localized on the cytosolic side. Two short stretches of amino acid sequences at the membrane proximal to the N- and C-termini (amino acids 61-71 and 299-303 respectively) were indispensable for Golgi localization of RKTG, but were not required for the interaction with Raf-1. The three loops facing the cytosol between the transmembrane domains had different roles in Golgi localization and Raf-1 interaction. While the first cytosolic loop was only important for Golgi localization, the third cytosolic loop was necessary for both Golgi localization and Raf-1 sequestration. Taken together, these findings suggest that RKTG is a type III membrane protein with its N-terminus facing the cytosol and multiple sequences are responsible for its localization at the Golgi apparatus and Raf-1 interaction. As RKTG is the first discovered Golgi protein with seven transmembrane domains, the knowledge derived from this study would not only provide structural information about the protein, but also pave the way for future characterization of the unique functions of RKTG in the regulation of cell signalling.

Identification of a redox-sensitive cysteine in GCP60 that regulates its interaction with golgin-160

Golgin-160 is ubiquitously expressed in vertebrates. It localizes to the cytoplasmic side of the Golgi and has a large C-terminal coiled-coil domain. The noncoiled-coil N-terminal head domain contains Golgi targeting information, a cryptic nuclear localization signal, and three caspase cleavage sites. Caspase cleavage of the golgin-160 head domain generates different fragments that can translocate to the nucleus by exposing the nuclear localization signal. We have previously shown that GCP60, a Golgi resident protein, interacts weakly with the golgin-160 head domain but has a strong interaction with one of the caspase-generated golgin-160 fragments (residues 140-311). This preferential interaction increases the Golgi retention of the golgin-160 fragment in cells overexpressing GCP60. Here we studied the interaction of golgin-160-(140-311) with GCP60 and identified a single cysteine residue in GCP60 (Cys-463) that is critical for the interaction of the two proteins. Mutation of the cysteine blocked the interaction in vitro and disrupted the ability to retain the golgin-160 fragment at the Golgi in cells. We also found that Cys-463 is redox-sensitive; in its reduced form, interaction with golgin-160 was diminished or abolished, whereas oxidation of the Cys-463 by hydrogen peroxide restored the interaction. In addition, incubation with a nitric oxide donor promoted this interaction in vitro. These findings suggest that nuclear translocation of golgin-160-(140-311) is a highly coordinated event regulated not only by cleavage of the golgin-160 head but also by the oxidation state of GCP60.

A novel 165-kDa Golgin protein induced by brain ischemia and phosphorylated by Akt protects against apoptosis

A cDNA encoding a novel protein was cloned from ischemic rat brain and found to be homologous to testis Mea-2 Golgi-associated protein (Golga3). The sequence predicted a 165-kDa protein, and in vitro translated protein exhibited a molecular mass of 165-170 kDa. Because brain ischemia induced the mRNA, and the protein localized to the Golgi apparatus, this protein was designated Ischemia-Inducible Golgin Protein 165 (IIGP165). In HeLa cells, serum and glucose deprivation-induced caspase-dependent cleavage of the IIGP165 protein, after which the IIGP165 fragments translocated to the nucleus. The C-terminus of IIGP165, which contains a LXXLL motif, appears to function as a transcriptional co-regulator. Akt co-localizes with IIGP165 protein in the Golgi in vivo, and phosphorylates IIGP165 on serine residues 345 and 134. Though transfection of IIGP165 cDNA alone does not protect HeLa cells from serum deprivation or Brefeldin-A-triggered cell death, co-transfection of both Akt and IIGP165 cDNA or combined IIGP165-transfection with PDGF treatment significantly protects HeLa cells better than either treatment alone. These data show that Akt phosphorylation of IIGP165 protects against apoptotic cell death, and add to evidence that the Golgi apparatus also plays a role in regulating apoptosis.

The anatomy of the cerebellar nuclei in the normal and scrambler mouse as revealed by the expression of the microtubule-associated protein kinesin light chain 3

Conventional kinesin is a motor protein complex including two heavy chains and two light chains (KLC). Junco et al. (Junco, A., Bhullar, B., Tarnasky, H.A. and van der Hoorn, F.A., 2001. Kinesin light-chain KLC3 expression in testis is restricted to spermatids. Biol. Reprod. 64, 1320-1330). recently reported the isolation of a novel KLC gene, klc3. In the present report, immunohistochemistry has been used to characterize the expression of KLC3 in the cerebella of normal and scrambler (scm) mutant mice. In cryostat sections through the cerebellum of the normal adult mouse immunoperoxidase stained for KLC3, reaction product is deposited in the nuclei and somata of deep cerebellar nuclear neurons. No other structures are stained in the cerebellum. Strong and specific KLC3 expression is observed in the adult cerebellum in all three major cerebellar nuclei--medial, interposed, and lateral. Double immunofluorescence studies reveal that KLC3 immunoreactivity is colocalized with both endosomes and GW bodies. KLC3 immunohistochemistry has been exploited to study the organization of the cerebellar nuclei in scrambler mice, in which disruption of the mdab1 gene results in severe foliation defects due to Purkinje cell ectopia, with most Purkinje cells clumped in centrally located clusters. Despite the severe failure of Purkinje cell migration, the cerebellar nuclei appear normal in scrambler mutant mice, suggesting that their topography is dependent neither on normal Purkinje cell positioning nor the Reelin signaling pathway.

The biogenesis of the Golgi ribbon: the roles of membrane input from the ER and of GM130

The Golgi complex in mammalian cells forms a continuous ribbon of interconnected stacks of flat cisternae. We show here that this distinctive architecture reflects and requires the continuous input of membranes from the endoplasmic reticulum (ER), in the form of pleiomorphic ER-to-Golgi carriers (EGCs). An important step in the biogenesis of the Golgi ribbon is the complete incorporation of the EGCs into the stacks. This requires the Golgi-matrix protein GM130, which continuously cycles between the cis-Golgi compartments and the EGCs. On acquiring GM130, the EGCs undergo homotypic tethering and fusion, maturing into larger and more homogeneous membrane units that appear primed for incorporation into the Golgi stacks. In the absence of GM130, this process is impaired and the EGCs remain as distinct entities. This induces the accumulation of tubulovesicular membranes, the shortening of the cisternae, and the breakdown of the Golgi ribbon. Under these conditions, however, secretory cargo can still be delivered to the Golgi complex, although this occurs less efficiently, and apparently through transient and/or limited continuities between the EGCs and the Golgi cisternae.

Golgin-160 Promotes Cell Surface Expression of the Beta-1 Adrenergic Receptor

Golgin-160 is a ubiquitously expressed peripheral Golgi membrane protein that is important for transduction of certain pro-apoptotic signals at the Golgi complex. However, the role of golgin-160 in normal Golgi structure and function is unknown. Here, we show that depletion of golgin-160 using RNA interference (RNAi) does not affect Golgi morphology or constitutive membrane traffic in HeLa cells. However, depletion of golgin-160 leads to significantly decreased cell surface levels of exogenously expressed beta1-adrenergic receptor (beta1AR), which can be rescued by expression of RNAi-resistant forms of golgin-160. Furthermore, overexpression of golgin-160 leads to higher surface levels of beta1AR. Golgin-160 is localized mostly in the cis and medial regions of the Golgi stack by immunoelectron microscopy, suggesting that it does not directly promote incorporation of beta1AR into transport vesicles at the trans Golgi network. Golgin-160 interacts with beta1AR in vitro, and we mapped the interaction to a region between residues 140 and 257 in the head of golgin-160 and the third intracellular loop of beta1AR. Our results support the idea that golgin-160 may promote efficient surface delivery of a subset of cargo molecules.

Modulation of D-serine levels via ubiquitin-dependent proteasomal degradation of serine racemase

Mammalian serine racemase is a brain-enriched enzyme that converts L- into D-serine in the nervous system. D-Serine is an endogenous co-agonist at the "glycine site" of N-methyl D-aspartate (NMDA) receptors that is required for the receptor/channel opening. Factors regulating the synthesis of D-serine have implications for the NMDA receptor transmission, but little is known on the signals and events affecting serine racemase levels. We found that serine racemase interacts with the Golgin subfamily A member 3 (Golga3) protein in yeast two-hybrid screening. The interaction was confirmed in vitro with the recombinant proteins in co-transfected HEK293 cells and in vivo by co-immunoprecipitation studies from brain homogenates. Golga3 and serine racemase co-localized at the cytosol, perinuclear Golgi region, and neuronal and glial cell processes in primary cultures. Golga3 significantly increased serine racemase steady-state levels in co-transfected HEK293 cells and primary astrocyte cultures. This observation led us to investigate mechanisms regulating serine racemase levels. We found that serine racemase is degraded through the ubiquitin-proteasomal system in a Golga3-modulated manner. Golga3 decreased the ubiquitylation of serine racemase both in vitro and in vivo and significantly increased the protein half-life in pulse-chase experiments. Our results suggest that the ubiquitin system is a main regulator of serine racemase and D-serine levels. Modulation of serine racemase degradation, such as that promoted by Golga3, provides a new mechanism for regulating brain d-serine levels and NMDA receptor activity.

Lysosomal localization of TRPML3 depends on TRPML2 and the mucolipidosis-associated protein TRPML1

Mucolipidosis type IV is an autosomal recessive lysosomal storage disorder characterized by severe neurodegeneration, achlorhydria, and visual impairments such as corneal opacity and strabismus. The disease arises due to mutations in a group 2 transient receptor potential (TRP)-related cation channel, TRPML1. Mammals encode two additional TRPML proteins named TRPML2 and TRPML3. Information regarding the propensity of these proteins to multimerize, their subcellular distribution and mechanisms that regulate their trafficking are limited. Here we demonstrate that TRPMLs interact to form homo- and heteromultimers. Moreover, the presence of either TRPML1 or TRPML2 specifically influences the spatial distribution of TRPML3. TRPML1 and TRPML2 homomultimers are lysosomal proteins, whereas TRPML3 homomultimers are in the endoplasmic reticulum. However, TRPML3 localizes to lysosomes when coexpressed with either TRPML1 or TRPML2 and is comparably mislocalized when lysosomal targeting of TRPML1 and TRPML2 is disrupted. Conversely, TRPML3 does not cause retention of TRPML1 or TRPML2 in the endoplasmic reticulum. These data demonstrate that there is a hierarchy controlling the subcellular distributions of the TRPMLs such that TRPML1 and TRPML2 dictate the localization of TRPML3 and not vice versa.

Preparation and characterization of recombinant golgin tethers

Golgin tethers are integral or peripheral Golgi proteins with predicted coiled-coil domains and many are known to interact directly with small GTPases of the Ypt/Rab or Arl families. Here we describe the preparation of recombinant golgins: GM130, p115 (and truncations thereof), the N-terminal fragment of giantin, CASP, and golgin-84.

Involvement of Golgin-160 in cell surface transport of renal ROMK channel: co-expression of Golgin-160 increases ROMK currents

The weak inward rectifier potassium channel ROMK is important for water and salt reabsorption in the kidney. Here we identified Golgin-160 as a novel interacting partner of the ROMK channel. By using yeast two-hybrid assays and co-immunoprecipitations from transfected cells, we demonstrate that Golgin-160 associates with the ROMK C-terminus. Immunofluorescence microscopy confirmed that both proteins are co-localized in the Golgi region. The interaction was further confirmed by the enhancement of ROMK currents by the co-expressed Golgin-160 in Xenopus oocytes. The increase in ROMK current amplitude was due to an increase in cell surface density of ROMK protein. Golgin-160 also stimulated current amplitudes of the related Kir2.1, and of voltage-gated Kv1.5 and Kv4.3 channels, but not the current amplitude of co-expressed HERG channel. These results demonstrate that the Golgi-associated Golgin-160 recognizes the cytoplasmic C-terminus of ROMK, thereby facilitating the transport of ROMK to the cell surface. However, the stimulatory effect on the activity of more distantly-related potassium channels suggests a more general role of Golgin-160 in the trafficking of plasma membrane proteins.

Holding it all together? Candidate proteins for the plant Golgi matrix

A combination of electron microscopy and fluorescence microscopy has provided us with a global picture of the structure of the plant Golgi apparatus. However, the components that shape this structure remain elusive. In other organisms, members of the golgin family of coiled-coil proteins are essential for Golgi structure and organisation. Putative Arabidopsis and rice homologues of some golgin family members can be identified using database searches. Likewise, the heterogeneous group of multi-subunit-tethering complexes is responsible for crucial transport steps that affect Golgi structure and cisternal organisation in animals and yeasts. The Arabidopsis genome harbours possible homologues for the majority of the subunits of these complexes, suggesting that they also operate in the plant kingdom.

Nuclear aggresomes form by fusion of PML-associated aggregates

Nuclear aggregates formed by proteins containing expanded poly-glutamine (poly-Q) tracts have been linked to the pathogenesis of poly-Q neurodegenerative diseases. Here, we show that a protein (GFP170*) lacking poly-Q tracts forms nuclear aggregates that share characteristics of poly-Q aggregates. GFP170* aggregates recruit cellular chaperones and proteasomes, and alter the organization of nuclear domains containing the promyelocytic leukemia (PML) protein. These results suggest that the formation of nuclear aggregates and their effects on nuclear architecture are not specific to poly-Q proteins. Using GFP170* as a model substrate, we explored the mechanistic details of nuclear aggregate formation. Fluorescence recovery after photobleaching and fluorescence loss in photobleaching analyses show that GFP170* molecules exchange rapidly between aggregates and a soluble pool of GFP170*, indicating that the aggregates are dynamic accumulations of GFP170*. The formation of cytoplasmic and nuclear GFP170* aggregates is microtubule-dependent. We show that within the nucleus, GFP170* initially deposits in small aggregates at or adjacent to PML bodies. Time-lapse imaging of live cells shows that small aggregates move toward each other and fuse to form larger aggregates. The coalescence of the aggregates is accompanied by spatial rearrangements of the PML bodies. Significantly, we find that the larger nuclear aggregates have complex internal substructures that reposition extensively during fusion of the aggregates. These studies suggest that nuclear aggregates may be viewed as dynamic multidomain inclusions that continuously remodel their components.

Identification of kinectin as a novel Behçet's disease autoantigen

There has been some evidence that Behçet's disease (BD) has a significant autoimmune component but the molecular identity of putative autoantigens has not been well characterized. In the initial analysis of the autoantibody profile in 39 Chinese BD patients, autoantibodies to cellular proteins were uncovered in 23% as determined by immunoblotting. We have now identified one of the major autoantibody specificities using expression cloning. Serum from a BD patient was used as a probe to immunoscreen a λZAP expression cDNA library. Candidate autoantigen cDNAs were characterized by direct nucleotide sequencing and their expressed products were examined for reactivity to the entire panel of BD sera using immunoprecipitation. Reactivity was also examined with normal control sera and disease control sera from patients with lupus and Sjögren's syndrome. Six independent candidate clones were isolated from the cDNA library screen and were identified as overlapping partial human kinectin cDNAs. The finding that kinectin was an autoantigen was verified in 9 out of 39 (23%) BD patient sera by immunoprecipitation of the in vitro translation products. Sera from controls showed no reactivity. The significance of kinectin as a participant in autoimmune pathogenesis in BD and the potential use of autoantibody to kinectin in serodiagnostics are discussed.

Golgins and GTPases, giving identity and structure to the Golgi apparatus

In this review we will focus on the recent advances in how coiled-coil proteins of the golgin family give identity and structure to the Golgi apparatus in animal cells. A number of recent studies reveal a common theme for the targeting of golgins containing the ARL-binding GRIP domain, and the related ARF-binding GRAB domain. Similarly, other golgins such as the vesicle tethering factor p115 and Bicaudal-D are targeted by the Rab GTPases, Rab1 and Rab6, respectively. Together golgins and their regulatory GTPases form a complex network, commonly known as the Golgi matrix, which organizes Golgi membranes and regulates membrane trafficking.

Golgi tethering factors

Transport of cargo to, through and from the Golgi complex is mediated by vesicular carriers and transient tubular connections. In this review, we describe vesicle tethering events with the understanding that similar events occur during transport via larger structures. Tethering factors can be generally divided into a group of coiled-coil proteins and a group of multi-subunit complexes. Current evidence suggests that these factors function in a variety of membrane-membrane tethering events at the Golgi complex, interact with SNARE molecules, and are regulated by small GTPases of the Rab and Arl families.

Transcriptional repression and cell death induced by nuclear aggregates of non-polyglutamine protein

Nuclear aggregates of polyglutamine (polyQ)-expanded proteins are associated with a number of neurodegenerative diseases including Huntington's disease (HD) and spinocerebellar ataxias (SCAs). The nuclear deposition of polyQ proteins correlates with rearrangements of nuclear matrix, transcriptional dysregulation, and cell death. To explore the requirement for polyQ tracks in educing such cellular responses, we examined whether a non-polyQ protein can deposit as nuclear aggregates and elicit similar responses. We report that a protein chimera (GFP170*) composed of the green fluorescent protein (GFP) fused to an internal fragment of the Golgi Complex Protein (GCP-170) forms nuclear aggregates analogous to those formed by polyQ proteins. Like the polyQ nuclear aggregates, GFP170* inclusions recruit molecular chaperones and proteasomal components, alter nuclear structures containing the promyelocytic leukemia protein (PML), recruit transcriptional factors such as CREB-binding protein (CBP) and p53, repress p53 transcriptional activity, and induce cell death. Our results indicate that nuclear aggregation and transcriptional effects are not unique to polyQ-containing proteins and may represent a general response to misfolded proteins in the nucleus.